[Core] Fix usage of @internal and @hidden tags

Please follow the following guidelines:

• use the @internal tag only for definitions that are not exported by src/index.ts and you do not want to generate documentation for them on docs.microsoft.com
• use the @hidden tag only for definitions that are exported by src\index.ts but you still do not want to generate documentation for them on docs.microsoft.com
• do not use both @internal and @hidden for one definition

[xirzec]

I’m leaning toward “leave those things hidden unless we have a good reason” since they’re all stuff that we’d not want external devs to see/think about

[ramya-rao-a]

Thanks all!